An electric discharge machine generally comprises a mechanical section, carrying a member for supporting a workpiece and mechanical components, and a power supply unit that by necessity must be positioned a physical distance away from the mechanical section. A train of controlled power pulses are produced in this power supply unit and applied to a gap between the tool electrode and the workpiece. As the inductance of a cable electrically connecting the power supply unit and the gap is large, the waveform of electrical current pulse generated in the gap suffers from increased distortion, thereby causing energy loss.
U.S. Pat. No. 5,750,951 discloses a transformer, for converting high frequency pulses transmitted from a power supply unit into high frequency alternating current, being provided as close as possible to the gap. The transformer disclosed in this patent comprises a ferrite ring core, a primary winding connected to a d.c. power source, and a secondary winding connected to the primary winding connecting a tool electrode to a workpiece. A case housing the transformer also houses a plurality of switches for connecting and disconnecting the transformer to and from a power supply circuit. When high frequency d.c. pulses are supplied to the gap, the transformer is disconnected from the power supply circuit, and when high frequency a.c. power is supplied to the gap, the transformer is connected to the power supply circuit. The case can be attached, for example, to a machining tank filled with dielectric fluid, or a work stand to which a workpiece is fixed.
FIG. 8 illustrates a conventional mechanical switch for use with a power supply circuit. The switch 100 comprises a case 101 formed of an insulating material, and a movable contact 102 and a fixed contact 102A for respectively electrically connecting to a first terminal C and a second terminal T. The movable contact 102 is comprised of a piston 102A and a piston rod 102B extending from a bottom surface of the piston 102A. A cylindrical hole 101A is formed in the case 101, and the piston 102A is provided so as to be capable of reciprocating movement within the case 101 by being guided by walls of the hole 101A. Openings 101B and 101C leading to the hole 101A are respectively formed in opposite side walls of the case 101. The piston rod 102B extends to the outside of the case 101 through an opening 101B in an axial direction of the piston 102A, and the first terminal C is attached to a tip of the piston rod 102B. An O ring type sealing member 105 is provided so that dielectric fluid does not encroach into the inside of the case from a gap between the piston rod 102B and the case 101. The fixed contact 104 is fitted into the case 101 and has a planar surface extending parallel to and opposite to the top planar surface of the piston 102A. Part of the fixed contact 104 extends to the outside of the case 101 water-tightly blocking the opening 101C, and the second terminal T is attached to a tip of the fixed contact 104. Holes 101F and 101G for supplying air for driving the movable contact 102 to the hole 101A are formed in the case 101. If the top surface of the piston 102A is brought into contact with the planar surface of the fixed contact 104 by supplying air from the hole 101G to the hole 101A, the first terminal C and the second terminal T are electrically connected. If the top surface of the piston 102A is taken away from the fixed contact 104 by supplying air from the hole 101F to the hole 101A, the first terminal C and the second terminal T are electrically disconnected.
During machining the switch 100 is immersed in dielectric fluid or dielectric fluid that has splashed out from the machining section is attached to the switch 100. Because of this, electrical current flows between the first and second terminals C and T, and the piston rod 102B is gradually corroded as a result of this electrolytic action. The sealing member 105 will also be damaged as a result of corrosion on the piston rod 102B. Additionally, fine metallic particles generated as a result of electric discharge machining become attached to the piston rod 102B, and these metallic particles may damage the sealing member 105. If the sealing member 105 is damaged in this way, dielectric fluid will encroach into the hole 101A, and the contacts 102 and 104 will be corroded. Also, encroachment of the dielectric fluid increases energy loss due to the occurrence of insulation deficiency of the switch.